Permeable and impermeable membranes have been used to separate ions, molecules, and solids from the liquid portion of the colloidal suspension or solution. Although filtration has been employed in this regard there is an ever present problem of plugging or fouling of the filter membrane. Methods of enhancing membrane permeate rates (dewatering rates) are found in the prior art. Such methods include the shearing of liquid slurry across the membrane in tangential flow i.e. crossflow filtration. This method uses a pump to force the feed slurry to flow tangentially to the dewatering membrane. The resulting sheer causes concentrated material, usually in the form of a filter cake, to be removed from the face of the membrane. Thus, the rate of liquid removal through the membrane is increased. Unfortunately, the provision of pumps to force the feed slurry in this manner requires expensive and bulky equipment and creates serious problems in the sealing of the vessel holding the colloidal suspension or solution.
Another method proposes the use of sonic vibration, created by ultrasonic transducers, to produce cavitation at the face of the membrane. A different technique proposes a shock-type system where the membrane support structure and a filtration apparatus are periodically shocked to induce the filter cake to drop from the membrane. A further process employs a shearing plate which is oscillated parallel to a fixed membrane. Further, an additional method teaches a system where a membrane is mechanically vibrated in a direction normal to the membrane. Alternatively, screening and sieving devices used in dry mineral and wet powder classification use screens vibrated parallel to the face of the screen to induce the powder to fall through the pores of the screen. None of these devices are suitable for separation of the components of a colloidal suspension or solution with the application of negative or positive pressure.
Ultrafilters manufactured by Millipore Corporation of Billerica, Mass. offer a system intended for separating proteins from aqueous solutions. The system utilizes a cylindrical probe which is inserted into a test tube containing the solution to be separated. The cylindrical wall of the probe is formed in part from a membrane material and the proteins pass through the filter from the solution occupying the annular volume between the probe and the test tube wall. The cylindrical probe is reciprocated over a small amplitude (less than 0.01 centimeters) and at 60 Hz. The shear created between the opposed walls of the probe and the test tube is partially effective in reducing plugging of the membrane by the proteins.
In general, the technique of cross-flow microfiltration and ultrafiltration is limited since shear rates above 20,000 sec−1 of intensity are difficult to achieve. Such high intensities require a massive amount of power and the provision of entrance pressures which are uneconomical. Also, such membranes are often arrayed in a rectangular pressure vessel such as a plate and frame cross-flow device. The transmembrane pressure drop is limited by the inherently weak vessel walls.
A membrane filtration device which is able to produce a large shear intensity on the exterior or face of the membrane simultaneously with the application of a large pressure drop across the membrane to create high permeate rates would be a great advance in the art of filtration and metal component separation.